﻿214 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  This 
  level, 
  therefore, 
  say, 
  4,000 
  feet 
  above 
  the 
  surface, 
  is 
  also 
  a 
  level 
  

   of 
  maximum 
  cloudiness. 
  

  

  Alto-cumulus 
  level. 
  — 
  The 
  strong 
  convection 
  common 
  to 
  dry 
  

   summer 
  weather 
  often 
  produces 
  small 
  cumulus 
  clouds 
  at 
  considerable 
  

   heights 
  — 
  frequently 
  2 
  to 
  2| 
  miles 
  above 
  the 
  surface. 
  Furthermore, 
  

   during 
  the 
  average 
  thunderstorm 
  a 
  great 
  deal 
  of 
  humidity 
  is 
  urged 
  

   up 
  to 
  roughly 
  this 
  same 
  level, 
  and 
  there 
  spread 
  out 
  into 
  a 
  wide 
  layer 
  

   by 
  the 
  swifter 
  winds 
  of 
  that 
  height. 
  Subsequent 
  convections 
  in 
  this 
  

   layer 
  often 
  induce 
  many 
  small 
  cumuli. 
  For 
  both 
  reasons, 
  therefore, 
  

   this 
  level, 
  known 
  as 
  the 
  alto-cumulus 
  level, 
  is 
  likewise 
  one 
  of 
  "maxi- 
  

   mum 
  frequency 
  of 
  cloud. 
  

  

  Cirro-stratus 
  level. 
  — 
  The 
  topmost 
  portions 
  of 
  the 
  clouds 
  in 
  a 
  

   cyclonic, 
  or 
  general, 
  rain 
  are 
  carried 
  forward 
  over 
  very 
  extensive 
  

   areas 
  by 
  probably 
  the 
  swiftest 
  winds 
  of 
  the 
  entire 
  atmosphere. 
  

   Hence 
  this 
  level, 
  roughly 
  5 
  miles 
  above 
  the 
  surface, 
  is 
  also 
  one 
  of 
  

   maximum 
  cloud 
  frequency, 
  or 
  at 
  least 
  of 
  cloud 
  observation. 
  

  

  Cirrus 
  level. 
  — 
  Rising 
  masses 
  of 
  air 
  can 
  not 
  ascend, 
  for 
  reasons 
  

   well 
  known, 
  beyond 
  the 
  level 
  of 
  the 
  highest 
  cirrus 
  clouds: 
  that 
  is, 
  

   in 
  middle 
  latitudes, 
  above 
  an 
  elevation 
  of 
  about 
  6 
  miles. 
  Any 
  

   air 
  that 
  reaches 
  this 
  level 
  necessarily 
  spreads 
  out 
  in 
  an 
  extensive 
  

   sheet 
  or 
  layer 
  in 
  which, 
  under 
  favorable 
  conditions, 
  condensation 
  

   occurs 
  in 
  the 
  form 
  of 
  fine 
  snow 
  crystals. 
  Hence 
  this 
  ultimate 
  

   level 
  of 
  the 
  ascending 
  air, 
  at 
  which 
  its 
  horizontal 
  spread 
  is 
  great, 
  is 
  

   also 
  one 
  of 
  maximum 
  cloud 
  extent. 
  

  

  LEVELS 
  OF 
  MINIMUM 
  CLOUDINESS. 
  

  

  If 
  there 
  are 
  different 
  levels 
  of 
  maximum 
  cloudiness, 
  it 
  follows 
  

   that 
  between 
  each 
  two 
  such 
  adjacent 
  levels 
  there 
  must 
  be 
  a 
  level 
  

   of 
  minimum 
  cloudiness. 
  However, 
  there 
  is 
  nothing 
  of 
  particular 
  

   interest 
  about 
  any 
  of 
  these 
  intermediate 
  levels. 
  

  

  Why 
  tJiere 
  are 
  no 
  clouds 
  above 
  the 
  high 
  cirrus. 
  — 
  The 
  region 
  above 
  

   the 
  high 
  cirrus 
  also 
  is 
  one 
  of 
  minimum 
  cloudiness. 
  In 
  fact 
  clouds 
  

   do 
  not 
  occur 
  in 
  that 
  region 
  at 
  all, 
  and 
  the 
  reason 
  is 
  as 
  follows 
  : 
  

  

  There 
  clearly 
  is 
  some 
  temperature 
  (actually 
  about 
  60° 
  F. 
  below 
  

   zero) 
  at 
  which 
  each 
  portion 
  of 
  the 
  high 
  atmosphere 
  must 
  lose 
  as 
  

   much 
  heat 
  by 
  radiation 
  as 
  it 
  gains, 
  and 
  this 
  is 
  the 
  lowest 
  tem- 
  

   perature 
  to 
  which 
  the 
  free 
  air 
  can 
  cool. 
  This 
  temperature 
  is 
  reached, 
  

   in 
  middle 
  latitudes, 
  at 
  about 
  6 
  to 
  7 
  miles 
  above 
  sea 
  level. 
  

   Hence 
  this 
  is 
  the 
  limit 
  of 
  vertical 
  convection 
  — 
  for 
  higher 
  convection 
  

   would 
  mean 
  lower 
  temperatures 
  — 
  and 
  the 
  maximum 
  height, 
  there- 
  

   fore, 
  to 
  which 
  water 
  vapor 
  is 
  carried. 
  Of 
  course 
  some 
  water 
  vapor 
  

   reaches 
  greater 
  heights 
  by 
  the 
  slow 
  process 
  of 
  diffusion, 
  but 
  all 
  of 
  

   this 
  region 
  is 
  subject 
  to 
  frequent 
  temperature 
  changes 
  just 
  as 
  is 
  the 
  

  

  